How can I make the shadows more defined?
A big light source is going to give  you very soft shadows. The bigger the source, the softer the shadows (think of the light on an overcast day, when the sun is diffused by clouds). A small light source (like direct sun or a street light) will make harder shadows.





Adding a second point light in front of the original source will give you defined shadows while keeping the light directon You might want to make the original soft source less bright)